On the Importance of Nanoparticle Necks and Carbon Impurities for Charge Trapping in TiO2.

Particle attachment and neck formation inside TiO2 nanoparticle networks determine materials performance in sensing, photo-electrochemistry, and catalysis. Nanoparticle necks can feature point defects with potential impact on the separation and recombination of photogenerated charges. Here, we investigated with electron paramagnetic resonance a point defect that traps electrons and predominantly forms in aggregated TiO2 nanoparticle systems. The associated paramagnetic center resonates in the g factor range between g = 2.0018 and 2.0028. Structure characterization and electron paramagnetic resonance data suggest that during materials processing, the paramagnetic electron center accumulates in the region of nanoparticle necks, where O2 adsorption and condensation can occur at cryogenic temperatures. Complementary density functional theory calculations reveal that residual carbon atoms, which potentially originate from synthesis, can substitute oxygen ions in the anionic sublattice, where they trap one or two electrons that mainly localize at the carbon. Their emergence upon particle neck formation is explained by the synthesis- and/or processing-induced particle attachment and aggregation facilitating carbon atom incorporation into the lattice. This study represents a substantial advance in linking dopants, point defects, and their spectroscopic fingerprints to microstructural features of oxide nanomaterials.

Single Metal Atoms on Oxide Surfaces: Assessing the Chemical Bond through 17O Electron Paramagnetic Resonance.

ConspectusEven in the gas phase single atoms possess catalytic properties, which can be crucially enhanced and modulated by the chemical interaction with a solid support. This effect, known as electronic metal-support interaction, encompasses charge transfer, orbital overlap, coordination structure, etc., in other words, all the crucial features of the chemical bond. These very features are the object of this Account, with specific reference to open-shell (paramagnetic) single metal atoms or ions on oxide supports. Such atomically dispersed species are part of the emerging class of heterogeneous catalysts known as single-atom catalysts (SACs). In these materials, atomic dispersion ensures maximum atom utilization and uniform active sites, whereby the nature of the chemical interaction between the metal and the oxide surface modulates the catalytic activity of the metal active site by tuning the energy of the frontier orbitals. A comprehensive set of examples includes fourth period metal atoms and ions in zeolites on insulating (e.g., MgO) or reducible (e.g., TiO2) oxides and are among the most relevant catalysts for a wealth of key processes of industrial and environmental relevance, from the abatement of NOx to the selective oxidation of hydrocarbons and the conversion of methane to methanol.There exist several spectroscopic techniques able to inform on the geometric and electronic structure of isolated single metal ion sites, but either they yield information averaged over the bulk or they lack description of the intimate features of chemical bonding, which include covalency, ionicity, electron and spin delocalization. All of these can be recovered at once by measuring the magnetic interactions between open-shell metals and the surrounding nuclei with Electron Paramagnetic Resonance (EPR) spectroscopy. In the case of oxides, this entails the synthesis of 17O isotopically enriched materials. We have established 17O EPR as a unique source of information about the local binding environment around oxygen of magnetic atoms or ions on different oxidic supports to rationalize structure-property relationships. Here, we will describe strategies for 17O surface enrichments and approaches to monitor the state of charge and spin delocalization of atoms or ions from K to Zn dispersed on oxide surfaces characterized by different chemical properties (i.e., basicity or reducibility). Emphasis is placed on chemical insight at the atomic-scale level achieved by 17O EPR, which is a crucial step in understanding the structure-property relationships of single metal atom catalysts and in enabling efficient design of future materials for a range of end uses.

Zinc oxide hollow spheres decorated with cerium dioxide. The role of morphology in the photoactivity of semiconducting oxides.

The photochemical activity of the recently proposed CeO2-ZnO photocatalytic material active under visible light has been improved by means of significant modifications of its morphology. A polymeric templating agent (Pluronic) has been used in the synthesis obtaining a particle morphology based on hollow spheres that is better defined in the case of high template concentration. The charge separation ability and the light-induced surface electron transfer under irradiation with visible polychromatic light in various ranges of wavelengths has been investigated by electron paramagnetic resonance. The reactivity of the photogenerated holes has been monitored by the spin trapping technique in the presence of DMPO. The hollow spheres morphology achieved through the synthesis here reported leads to systems with a higher photoactivity under visible irradiation than the same system displaying the classic platelets morphology. A parallel increase of the photocatalytic activity of this novel system in pollution remediation reactions is therefore predictable.

Nitrogen-doped semiconducting oxides. Implications on photochemical, photocatalytic and electronic properties derived from EPR spectroscopy.

Engineering defects in semiconducting metal oxides is a challenge that remains at the forefront of materials chemistry research. Nitrogen has emerged as one of the most attractive elements able to tune the photochemical and photocatalytic properties of semiconducting oxides, boosting visible-light harvesting and charge separation events, key elements in promoting solar driven chemical reactions. Doping with nitrogen is also a strategy suggested to obtain p-type conduction properties in oxides showing n-type features in their pristine state and to impart collective magnetic properties to the same systems. Here, we review the evolution in the understanding of the role of nitrogen doping in modifying the photochemical and electronic properties of the most common semiconducting oxides used in mentioned applications including: TiO2, ZnO, SnO2 and zirconium titanates. With an emphasis on polycrystalline materials, we highlight the unique role of Electron Paramagnetic Resonance (EPR) spectroscopy in the direct detection of open-shell N-based defects and in the definition of their structural and electronic properties. Synthetic strategies for the insertion of nitrogen defects in the various matrices are also discussed, along with the influence of the corresponding low-lying energy states on the general electronic properties of the doped solids.

Electron magnetic resonance in heterogeneous photocatalysis research.

The contribution of electron magnetic resonance techniques, and in particular of CW-EPR, to the experimental research on photocatalytic phenomena is illustrated in this paper with selected examples. In the first part of the paper the role of EPR in unravelling the nature and the features of extrinsic point defects in semiconducting oxides is epitomized using the important example of the photoactive nitrogen center in various semiconducting oxides. In the second part we describe how EPR can monitor the processes that follow the initial photoinduced charge separation in photocatalysis, namely the stabilisation, migration and surface reactivity of electrons and holes. Finally, we will discuss how the role of EPR in photocatalysis is not limited to monitor phenomena occurring in the solid or at its surface but it can be extended to the investigation of the liquid phase by employing the spin trapping techniques to monitor the nature and the concentration of the reactive free radicals formed along the photocatalytic process.

Nature and Topology of Metal-Oxygen Binding Sites in Zeolite Materials: 17 O High-Resolution EPR Spectroscopy of Metal-Loaded ZSM-5.

Determining structural models is pivotal to the rational understanding and development of heterogeneous catalytic systems. A paradigmatic case is represented by open-shell metals supported on oxides, where the catalytic properties crucially depend on the nature of the metal-oxygen bonds and the extent of charge and spin transfer. Through a combination of selective 17 O isotopic enrichment and the unique properties of open-shell s-state monovalent Group 12 cations, we derive a site-specific topological description of active sites in an MFI zeolite. We show that just a few selected sites out of all possible are populated and that the relative occupancies depend on the specific properties of the metal, and we provide maps of charge and spin transfer at the metal-oxygen interface. This approach is not restricted to zeotype materials, rather it is applicable to any catalysts supported on oxygen-containing materials.

Insights Into the Sunlight-Driven Water Oxidation by Ce and Er-Doped ZrO 2.

In the present work, the activity of Ce and Er-doped ZrO2 nanopowders for sun-driven photocatalytic water oxidation has been investigated. ZrO2 powders with tunable amounts of tetragonal, monoclinic and cubic polymorphs have been synthesized by introducing Ce and Er (from 0.5 to 10 mol % on an oxide basis) through hydrothermal method. The aim of this work is to investigate the role of rare earth (RE) ions rich of electrons (Er3+) and with entirely empty levels (Ce4+) in the ZrO2 matrix for the sun-driven photocatalytic water oxidation reaction. The samples have been characterized by means of UV-Vis spectroscopy, X-Ray diffraction (XRD), N2 adsorption, X-ray photoelectron spectrophotometry (XPS) and transmission electronic microscopy (TEM) with energy dispersive spectroscopy (EDS). With respect to the bare ZrO2 mainly containing monoclinic (m-) phase, an increasing amount of rare-earth (RE) dopant was found to improve the specific BET surface area and to stabilize the tetragonal (t-) or cubic (c-) polymorphs of ZrO2 at room temperature. XRD data confirmed that dopants were mainly inserted in the t-ZrO2 phase. The photocatalytic O2 evolution from water under AM 1.5 G simulated sunlight illumination of the prepared samples have been correlated with their optical, structural and chemical properties. The effect of the dopant concentration on the chemical-physical and photocatalytic properties of the Er- and Ce-doped ZrO2 materials was elucidated. The samples with 5% of RE oxide were the most active, i.e., three times more than pure zirconia. Their superior photocatalytic activity was found to be mainly correlated to two factors: (i) an optimal surface concentration of RE ions of about 3.7%, which increased charge carriers separation in the photocatalysts surface due more superficial defects of the t-ZrO2 and a higher surface area, thus enhancing the reaction kinetics, (ii) a controlled amount of monoclinic vs. tetragonal (or cubic) polymorphs of zirconia with an optimum ratio of about 70/30 of t-ZrO2/m-ZrO2. Instead, the increased ability of the RE-doped ZrO2 to harvest visible light was found to have a secondary role on the photocatalytic activity of the Ce-doped ZrO2 material.

Copper-Modified TiO2 and ZrTiO4: Cu Oxidation State Evolution during Photocatalytic Hydrogen Production.

In the present work, two H2 evolution photocatalysts were prepared by employing two different oxides, TiO2 and zirconium titanate (ZrTiO4), as the support of various copper phases. For both the supports the same Cu loading (0.5% w/w) was adopted, but two different impregnation procedures have been followed, leading to different forms of Cu in the final composite material that are: (i) Cu(II) species dispersed on the oxide surface and (ii) Cu2O particles dispersed on the oxide surface. The present paper based on the parallel use of photocatalytic test and spectroscopic analysis performed in catalytic conditions illustrates the evolution of photocatalytic systems occurring during the H2 evolution reaction tests, pointing out that the as-prepared materials represent a pre-catalyst and they are modified during irradiation leading to the real working systems different from the starting ones. The herein presented spectroscopic analysis aims to contribute to the living debate on the oxidation state of copper in mixed Cu/oxide materials and on its role in hydrogen evolution under photocatalytic conditions.

The Existence of Nitrate Radicals in Irradiated TiO2 Aqueous Suspensions in the Presence of Nitrate Ions.

Evidence of the existence of nitrate radical in irradiated aqueous TiO2 suspensions in the presence of nitrate ions are reported for the first time. The joint use of UV/Vis and EPR spectroscopy showed that nitrate radicals are formed by hole induced oxidation of nitrate ions. Photocatalytic degradation of a model alkene compound allowed to highlight the presence of an intermediate organic nitrate deriving from nitrate radical attack to the double bond of the substrate. These results not only allow deeper understanding of photocatalytic processes, but open the route to new green photocatalytic syntheses initiated by nitrate radicals and to new insights in the field of atmospheric chemistry.

Photoactivity properties of ZnO doped with cerium ions: an EPR study.

In the present study, we investigated the effect of cerium doping in zinc oxide matrix, used as photoactive material. Cerium ions into the matrix of ZnO can act like a 'trap' for the electrons, lowering the e -/h + recombination rate and so increasing the photocatalytic efficiency of the ZnO. We synthesised doped samples using a simple precipitation route. The amount of dopant used was, 1 and 10% molar. The samples have been studied via x-ray diffraction measurements for the structural characterisation; UV-visible diffuse reflectance was used for the optical analysis; Branauer-Emmett-Teller (BET) model for the measurement of the surface area. Finally, the samples have been analysed via electron paramagnetic resonance (EPR) spectroscopy for the electronic characterisation and for testing their photoactivity. The spin trapping technique was also use to measure the amount of stable radical adducts formed via reaction of OH• radicals with molecules of the DMPO (5,5-dimethyl-1-pyrroline-N-oxide) spin probe.

EPR approaches to heterogeneous catalysis. The chemistry of titanium in heterogeneous catalysts and photocatalysts.

Paramagnetic species are often involved in catalytic or photocatalytic reactions occurring at the solid-gas interface of heterogeneous catalysts. In this contribution we will provide an overview of the wealth and breadth of information that can be obtained from EPR in the characterization of paramagnetic species in such systems, illustrating the advantages that modern pulsed EPR methodologies can offer in monitoring the elementary processes occurring within the coordination sphere of surface transition-metal ions. To do so we selected three representative systems, where titanium ions in low oxidation states act as active catalytic sites, trying to outline the methodological approaches which characterize the application of EPR techniques and the questions that can be answered and addressed relative to the characterization of heterogeneous catalytic materials.

Ferromagnetic Interactions in Highly Stable, Partially Reduced TiO2 : The S=2 State in Anatase.

We report direct evidence for quintuplet spin states in a particular kind of reduced TiO2 anatase obtained by the mild oxidation of TiB2 under hydrothermal conditions. Continuous-wave and pulse EPR spectroscopy at X and Q band frequencies provide compelling evidence for the presence of S=2 states, stable in a wide range of temperatures up to room temperature. A tentative model, corroborated by spin-polarized DFT calculations, is proposed, which consists of four ferromagnetically interacting Ti3+ ions with distances ranging from 0.5 nm to 0.8 nm and tetrahedral arrangement.

Effect of sildenafil on human aromatase activity: From in vitro structural analysis to catalysis and inhibition in cells.

Aromatase catalyses the conversion of androgens into estrogens and is a well-known target for breast cancer therapy. As it has been suggested that its activity is affected by inhibitors of phosphodiesterase-5, this work investigates the potential interaction of sildenafil with aromatase. This is carried out both at molecular level through structural and kinetics assays applied to the purified enzyme, and at cellular level using neuronal and breast cancer cell lines. Sildenafil is found to bind to aromatase with a KD of 0.58±0.05µM acting as a partial and mixed inhibitor with a maximal inhibition of 35±2%. Hyperfine sublevel correlation spectroscopy and docking studies show that sildenafil binds to the heme iron via its 6th axial water ligand. These results also provide information on the starting molecular scaffold for the development of new generations of drugs designed to inhibit aromatase as well as phosphodiesterase-5, a new emerging target for breast cancer therapy.

Mechanism of the Cyclo-Oligomerisation of C2 H2 on Anatase TiO2 (101) and (001) Surfaces and Their Reduction: An Electron Paramagnetic Resonance and Density Functional Theory Study.

Dehydroxylated, hydroxylated and hydrated anatase TiO2 samples have been exposed to acetylene at room temperature. The interaction leads to the formation of polycyclic aromatic hydrocarbons (PAHs) and is accompanied by the appearance of Ti3+ ions, as shown by electron paramagnetic resonance (EPR) spectra. Fully or partly dehydroxylated samples show higher reactivity, whereas the hydrated samples are chemically inert. The experimental results point towards a crucial role of the more reactive (001) facets of anatase nanoparticles. Density functional theory calculations show that acetylene physisorbs on the anatase (101) surface without activation of the C-H bond. The reduced (101) surface (O vacancies) leads to acetylene activation but not to dissociative adsorption. In contrast, the dehydroxylated (001) anatase surface is very active and leads to the spontaneous splitting of the C-H bond with formation of Ti-C2 H and OH groups. This is followed by subsequent additions of C2 H2 molecules with formation of PAHs. During the dissociation of C2 H2 , radical species do not form and electrons are not transferred to the surface because direct Ti-C covalent bonds form on the surface. However, the ring closure in the formation of the aromatic compounds leaves behind hydrogen atoms that donate their valence electrons to the oxide. This results in the appearance of EPR-active Ti3+ centres.

The effect of a C298D mutation in CaHydA [FeFe]-hydrogenase: Insights into the protein-metal cluster interaction by EPR and FTIR spectroscopic investigation.

A conserved cysteine located in the signature motif of the catalytic center (H-cluster) of [FeFe]-hydrogenases functions in proton transfer. This residue corresponds to C298 in Clostridium acetobutylicum CaHydA. Despite the chemical and structural difference, the mutant C298D retains fast catalytic activity, while replacement with any other amino acid causes significant activity loss. Given the proximity of C298 to the H-cluster, the effect of the C298D mutation on the catalytic center was studied by continuous wave (CW) and pulse electron paramagnetic resonance (EPR) and by Fourier transform infrared (FTIR) spectroscopies. Comparison of the C298D mutant with the wild type CaHydA by CW and pulse EPR showed that the electronic structure of the center is not altered. FTIR spectroscopy confirmed that absorption peak values observed in the mutant are virtually identical to those observed in the wild type, indicating that the H-cluster is not generally affected by the mutation. Significant differences were observed only in the inhibited state Hox-CO: the vibrational modes assigned to the COexo and Fed-CO in this state are shifted to lower values in C298D, suggesting different interaction of these ligands with the protein moiety when C298 is changed to D298. More relevant to the catalytic cycle, the redox equilibrium between the Hox and Hred states is modified by the mutation, causing a prevalence of the oxidized state. This work highlights how the interactions between the protein environment and the H-cluster, a dynamic closely interconnected system, can be engineered and studied in the perspective of designing bio-inspired catalysts and mimics.

Probing the coordinative unsaturation and local environment of Ti³âº sites in an activated high-yield Ziegler-Natta catalyst.

The typical activation of a fourth generation Ziegler-Natta catalyst TiCl4/MgCl2/phthalate with triethyl aluminum generates Ti(3+) centers that are investigated by multi-frequency continuous wave and pulse EPR methods. Two families of isolated, molecule-like Ti(3+) species have been identified. A comparison of the experimentally derived g tensors and (35,37)Cl hyperfine and nuclear-quadrupole tensors with DFT-computed values suggests that the dominant EPR-active Ti(3+)  species is located on MgCl2(110) surfaces (or equivalent MgCl2 terminations with tetra-coordinated Mg). O2 reactivity tests show that a fraction of these Ti sites is chemically accessible, an important result in view of the search for the true catalyst active site in olefin polymerization.

On the redox mechanism operating along C2H2 self-assembly at the surface of TiO2.

The interaction of acetylene with the TiO2 surface at room temperature entails a complex set of self-assembly reactions with the formation of products having relatively high molecular weight. In a previous paper by some of us (Jain, S. M.; et al. J. Mater. Chem. A 2014, 2, 12247-12254), the C2H2-TiO2 reaction has been monitored, essentially by Fourier transform infrared spectroscopy, at the surface of P25 (a mixture of anatase and rutile, typical benchmark material in the field of photocatalysis) in order to elucidate the nature of the products of this surface reaction. In the present paper, the same process was followed, for the first time, using electron paramagnetic resonance (EPR) and monitoring by the thermogravimetric analysis the weight loss of the material upon heating in order to further investigate the complex mechanism of the surface reaction. This was done using pure anatase and comparing the EPR results with those concerning both rutile and P25. The self-assembly mechanism occurring at the interface is accompanied by the formation of EPR visible Ti(3+) centers due to electrons injection in the TiO2 substrate. This finding clarifies that at least one of the reaction channels of this complex process (namely, the formation of polycyclic aromatic hydrocarbons) is based on the heterolytic dissociative chemisorption of acetylene, followed by a redox interaction between the adsorbate and the solid, which allows the creation of the building blocks necessary to assemble polyaromatic molecules.

The interaction of oxygen with the surface of CeO2-TiO2 mixed systems: an example of fully reversible surface-to-molecule electron transfer.

The interaction of oxygen with the surface of CeO2-TiO2 mixed oxides prepared via sol-gel was investigated by means of electron paramagnetic resonance (EPR). Upon admission of molecular oxygen onto the surface of the as prepared materials (which underwent final oxidative calcination) the formation of superoxide O2(-) ions is observed without the need for preliminary annealing in a vacuum and consequent oxygen depletion. The superoxide species is symmetrically adsorbed ("side-on" structure) on the top of a Ce(4+) ion. Surprisingly the electron transfer is fully reversible at room temperature having the typical behavior shown by molecular oxygen carriers, which, however, link to oxygen in a completely different manner ("end-on" structure). We suggest that the active sites are Ce(3+) ions present in the stoichiometric cerium titanate which forms during the synthesis. The features of these Ce(3+) ions must be different from those of the same ions formed in CeO2 by reductive treatments, which show a different reactivity to O2. The observation reported here opens up innovative perspectives in the field of heterogeneous catalysis and in that of sensors as the total reversibility of the electron transfer is observed in a significant range of oxygen pressure.

Probing the redox chemistry of titanium silicalite-1: formation of tetrahedral Ti3+ centers by reaction with triethylaluminum.

Transition-metal ions with open-shell configurations hold promise in the development of novel coordination chemistry and potentially unprecedented redox catalysis. Framework-substituted Ti(3+) ions with tetrahedral coordination are generated by reductive activation of titanium silicalite-1 with triethylaluminum, an indispensable co-catalyst for heterogeneous Ziegler-Natta polymerization catalysts. Continuous-wave and pulse electron paramagnetic resonance methods are applied to unravel details on the local environment of the reduced transition metal-ions, which are shown to be part of the silica framework by detection of (29)Si hyperfine interactions. The chemical accessibility of the reduced sites is probed using ammonia as probe molecule. Evidence is found for the coordination of a single ammonia molecule. Comparison to similar systems, such as TiAlPO-5, reveals clear differences in the coordination chemistry of the reduced Ti sites in the two solids, which may be understood considering the different electronic properties of the solid frameworks.